The present invention relates to a glass composition suitable for making glass, especially flat glass, that exhibits high visible light transmission and low heat transmission.
Use of glass to glaze vehicles and buildings is currently on the increase as the desire and demand for more visible light in enclosed spaces increases. However this is alongside more stringent demands in terms of energy efficiency, for example to reduce the load on air-conditioning systems in vehicles and buildings and providing lighter-weight glazings for vehicles. A number of avenues have been explored to create glazings that meet these so-called “high performance” requirements of high light transmission and low heat transmission; many of the solutions proposed to date focus on laminated glazings (i.e. glazings comprising two or more panes of glazing material joined together by an interlayer ply extending between each).
One way is to provide a laminate with a solar control coating (often silver-based) that permits transmission of visible light whilst preferentially reflecting infrared radiation. Another option is to use a specialist interlayer material, such as a specialist polyvinyl butyral, that has been specifically designed to maximise light transmission and minimise heat transmission through it. Unfortunately such solutions are relatively expensive to provide, and in the case of deposition of a coating on a substrate, result in the introduction of a further processing step. Furthermore, such solutions are not applicable to monoliths (i.e. single panes of glass).
It has therefore been a task for the glass industry over recent years to design and manufacture glasses that in themselves have high visible light transmission (i.e. greater than 70% when measured with CIE Illuminant A) and low direct solar heat transmission (i.e. less than 50% when measured according to ISO 9050; Air Mass 1.5) at least one thickness in the range 1 mm to 10 mm. Such glasses should be useful as monoliths and when incorporated into laminates and multiple pane glazing units (i.e. glazings having two or more panes of glass separated by a gaseous layer in a sealed space between each).
As a starting point, clear flat glass, especially clear float glass, is typically made using the batch ingredients sand, soda ash, limestone, dolomite, saltcake and cullet, which may result in a glass containing for example 70-73% SiO2, 12-14% Na2O, 7.5-10% CaO, 3-5% MgO, 0-2% Al2O3, 0-1% K2O, 0-0.3% SO3 and 0.07-0.13% Fe2O3 (total iron) which will ordinarily be present in both its oxidised Fe(III) and reduced Fe(II) form, and having a light transmission of around 89% and a direct solar heat transmission of around 83%, both at 3.85 mm.
There have been many suggestions and proposals for how to achieve high performance glasses by modification of typical flat glass compositions. Broadly speaking, these solutions fall into one of two general categories. On the one hand, it has been suggested that to improve the infrared absorption of a glass (to reduce its heat transmission), the total iron content of the glass should be increased. For a given ferrous ratio (i.e. the proportion of ferrous Fe(II) ions to ferric Fe(III) ions), this increases the amount of both ferrous iron and ferric iron. Ferrous iron is a known absorber of infrared radiation and a typical ferrous level is approximately 25%. However, heat is absorbed at the expense of transmission of visible light, which may be reduced below a level which is acceptable for automotive purposes—in Europe there are currently legal requirements stipulating that, for example an automotive windscreen must have a visible light transmission greater than 75% (measured with CIE Illuminant A), and similar legislation exists elsewhere.
Further to this proposal, it has been suggested that for a given total iron content, the ferrous ratio should be modified to increase the proportion of ferrous iron in a glass so as to preferentially increase its heat absorbing capacity. To manufacture a higher ferrous glass (having a ferrous level of around 30% or above), reducing conditions in a glass melting furnace are required. One way of achieving these conditions is to add carbon to the batch ingredients. Unfortunately, with highly reducing conditions, sulphur (as sulphate) which is present in the batch ingredients as a refining aid, along with iron generates amber colouration in the glass (which reduces visible light transmission). Sulphates also react with the added carbon; this introduces silica faults (often observed as un-melted silica) into the glass, which is a severe manufacturing problem.
Furthermore, the resultant molten glass is so absorptive of heat that it is extremely difficult for any of the heat incident on the surface of the glass to penetrate into the glass body below. Such reduced thermal efficiency leads to a glass that is poorly melted and refined and thus of unacceptable quality.
EP 0 297 404 A1 describes a solution to these problems which involves redesigning a glass-melting furnace based on vacuum-refining techniques such that melting and refining of molten glass each occur in discrete, sequential stages. As one might imagine however, such a radical proposal is costly and complex to implement.
On the other hand, it has been proposed to shift the ferrous iron absorption band to longer wavelengths. In typical iron-containing glass, the major ferrous iron absorption band extends from around 550 to 1600 nm, and has its peak at around 1050 nm. The spectrum of visible light extends between 380 and 770 nm; the longer wavelength end of which is overlapped by the ferrous iron absorption band. By shifting the peak of this band to around 1150 nm, its overlap with the visible part of the spectrum is reduced, thus increasing the amount of visible light transmitted by a glass having a given iron content and ferrous level. To date, this appears to be one of the more popular routes to achieving a high performance glass, and there have been many proposals as to the exact manner in which it should be done.
Japanese Patent Publication 60-215546-A describes a glass which comprises typical glass constituents of silica, alumina, etc. along with baria (BaO). In its exemplary glasses, the amount of SiO2 is less than 70%, Al2O3 less than 1.5%, CaO greater than 5% and MgO less than 2%. In addition these glasses contain high BaO (greater than 7%) and either typical Na2O levels (approximately 12-13%) but no K2O, or reduced Na2O (approximately 6%) and high K2O (9-10%). The combination of reduction of the amount of MgO and inclusion of between 4 and 15% BaO are factors that shift the ferrous iron absorption band to longer wavelengths.
Unfortunately, such reduction (or complete removal) of MgO often leads to an increase in the liquidus temperature of the glass. The physical effect of this is an increase in highly undesirable occurrences of devitrification (when solid glass forms in the molten glass body in the working end of a glass-melting furnace). Not only does devitrification cause problems for the glass that is currently being melted in terms of its quality, it may also cause problems for the next glass to be melted because the different liquidus temperature of this next molten glass may permit re-melting of the devitrification, resulting in its contamination.
Furthermore, the combined effect of Na2O reduction and K2O increase often leads to other melting problems. K2O effectively “traps” carbon dioxide (a by-product of the batch ingredient melting reactions) in the molten glass, which can often be observed subsequently “erupting” from the surface of the melt, leading to unsatisfactory glass quality.
EP 0 629 179 A1 describes a glass comprising 69-75% SiO2, 0-3% Al2O3, 2-10% CaO, 0-2% MgO, 9-17% Na2O, 0-8% K2O, 0.2-1.5% Fe2O3 and less than 4% BaO to achieve high performance and specifically an infrared absorption band with a peak situated at a wavelength greater than approximately 1100 nm. Similar melting problems due to low MgO (or its complete absence) and high K2O contents may again be observed. Furthermore in contradiction to the teaching of JP '546 above, in EP '179 inclusion of less than 4% BaO also appears to have the effect of shifting the ferrous iron absorption band to longer wavelengths.